Follow these steps:
1) Copy showcase-child directory from inside showcase/themes to wp-content/themes directory and activate it. Go to Appearance -> Menus and set your desired Primary menu.
2) Create functions.php in showcase-child directory and paste this code:
<?php add_image_size('index-thumbnail', 298, 244, true);
3) Copy showcase/loop.php to showcase-child/loop.php and edit it.
<?php the_post_thumbnail(); ?>
<?php the_post_thumbnail('index-thumb'); ?>
Refresh your site’s homepage and you should see clearer and correctly sized featured images. If not, regenerate thumbnails.